How to choose a size/resolution for the sprites used as game assets in Unity2D ?

Hello am new to Unity and I have started working with the 2D system of it.
and my main focus is at the android/iOS devices.

So what size/resolution of the sprites should I prefer keeping in mind that at last the device can be a 10-inch tablet!

From what im doing right now i would personally do 256 by 256. But hey it all depends on your style really. Sorry if this doesnt help but stuff like this can very from a wide range of choices.

i got it solved I prepare large images and then change Pixel to Unit ratio in Unity.
set the max size to 1024 and uncheck mim maps checkbox. It gives me the dimensions i want, the image is quite clear(sharpen it a little in Pshop) and also saves memory.